Ultra high frequency oscillator



Feb. 22, 1949. J. H. FREMLIN 2,462,085

ULTRA HIGH FREQUENCY OSCILLATOR Filed Dec. 2, 1943 5 Sheets-Sheet l ATZUHWY Feb. 22, 1949. J. H. FREML|N ULTRA HIGH FREQUENCY OSILLATOR 5 sheets-sheet 2 Filed Deo. 2, 1943 Feb. 22, 4949. J. H. FRI-:MUNv

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Patented Feb. 22, 1949 UNITED STATES TENT OFFICE ULTRA HIGH FREQUENCY OSCILLATUR corporation of Delaware Application December 2. 1943, Serial No. 512,577 In Great Britain December 4, 1942 (Sl. Z50-27.5)

7 Claims.

The present invention relates to ultra high frequency oscillation generators.

It is well known that at very short wave-lengths it is extremely difficult to obtain high power oscillators of suii'icient stability, that is, whose frequency variation is small compared to the frequency band width to be used by an associated receiver. Thus even at lO cm. wave-length the frequency of a transmitter may decrease by four or five megacycles per second after switching on and by additional amounts if the impedance of the load circuit changes, for example, on account of objects moving in front of an aerial couped to the oscillator. At'3 cm. wave-length this latter effect may be very serious as a reection of 1% of the power radiated back down the antenna feeder may shift the frequency as much as 6 megacycles per second. It is very desirable to have a local oscillator in a receiver co-operating with such a transmitter, in which receiver the frequency can be rapidly controlled in such a way as to maintain the intermediate frequency constant in spite of these variations. A controlling voltage derived in dependence upon the frequency divergence from the normal value provides a convenient controlling medium.

It is therefore an object of this invention to provide an ultra high frequency oscillator whose frequency can be controlled by a controlling voltage.

Oscillation generators of frequencies in the frequency range desired are of the velocity modulation type. Such devices comprise an electron stream or beam which is modulated in velocity by a high frequency electric field and then passed through a field free space during the passage through which the electrons become grouped or "bunched and then again through an electric field of the same high frequency in suitable phase so that it absorbs energy from the groups of electrons. The modulating field and the absorbing eld may be diieren-t parts of the same field or the two fields may be coupled together, energy being fed from the energy absorbing field tothe modulating field. thereby maintaining the fields in oscillation. The iieds are usually contained within a space bounded by conducting walls, and will be herein called a resonant chamber or resonator, and must be of dimensions suitable for resonance at a desired frequency. It will be apparent that, for oscillations to take place, the velocity of the electrons in a beam exciting such a. resonator must have a specific value, and this velocity depends upon the voltage applied to acceleratev the electrons. In many of the forms of oscillation generators of the velocity modulation type hitherto known a smal? variation in the accelerating voltage which enables the oscillator to notion results in the oscillator ceasing to function, the characteristic curve showing the relation between acceerating voltage and oscillator output being a very steep peaked curve.

According to the present invention there is proided an electron discharge apparatus comprising, in combination, an electron discharge device of the velocity modulation type having a resonant chamber system and an antenna having a length less than one quarter of the minimum wave length at which the apparatus is to operate, said antenna being mounted within the envelope of said device and on said system so as to be excited by said system.

The invention will be made clearer in the following description taken in conjunction with the accompanying drawings in which Figures 1, 2 and 3 are explanatory diagrammatic showings of the general features of the invention.

Figure 4 is a longitudinal section of a practical embodiment of the invention.

Figure 5 is a cross section on an enlarged scale taken along the line 5-5 in Figure 4.

Figure 6 is a perspective view of the device shown in Figures 4 and 5.

Figure 'l is a fragmentary perspective view of the device used with a wave guide.

Figure 8 shows an alternative construction and Figure 9 is an exploded view of part of the construction shown in Figure 8.

Fig. 1f) is a longitudinal cross-section of a practical embodiment of the construction shown in Figs. 8 and 9;

Fig. ll is a longitudinal cross-section taken along the line ll-ll of Fig. 10; and

Fig. l2 shows a detail of the tube shown in Fig. 6.

Figure 1 of .the drawings shows diagrammatically a suitable form of circuit for use with an electron beam, and whose variation of frequency with change in electron accelerating voltage is large compared to that of the sharply resonant cavities often used at centimeter wavelengths. The reference I designates a short length of coaxial transmission line short circuited at one end and loaded with a small capacity 2 at the other end. The centre line is extended by an antenna 3, the length of which may be considerably less than a quarter of the oscillation wave- 3 a reector plate 1i. The 'loading capacity 2 is not an essential part of the circuit but itis usually present in a tube that can be excited efficiently by a velocity modulated electron stream.

The antenna 3 is, however, of greater irnportance. As this antenna increases in length, the frequency change that can be obtained increases, so long as the length is small compared to one quarter i, but the loading on the circuit does likewise and it is, therefore, useful to choose the shortest length which will give the desired frequency change. To obtain a variation oi frequency with the excitation by an electron stream, the antenna length may for example be about 0.07 (Then the electrical length of the coaxial line will be about 0.l6.\.) The antenna diameter has but a slight effect; in a particular case it was about 0005A.

It will be observed that the antenna forms with the outer conductor a reactance which is connected across the physically open end ol the resonant chamber so that this latter can resonate and is not too highly damped by radiation from the antenna.

The excitation of the circuit is by an electron beam as stated hereinbefore and illustrated in Figures 2 and 3 which are diagrammatic showings corresponding respectively to transverse and longitudinal sections of the coaxial line. This beam is produced by an electron gun in known manner and shown as cathode II and focussing or accelerating electrode I2: Such a device without the antenna is described fully in the U. S. Patent No. 2,320,860. In brief, a slot is cut through both outer and inner conductors so that electrons traversing the system are affected by the high frequency eld in two gaps, 5 and ii separated by a drift space 'I which occurs in the interior of the central conductor of the coaxial line. It can now be seen how the loading capacity 2 of Figure 1 is formed by the ns 8 and 9 which reduce the width of the gaps 5 and 6 to values reasonably small compared to the distance travelled by an electron in one cycle at the voltage and frequency concerned, and which also prevent appreciable loss by radiation from the. slots in the outer conductor. In the simplest mode of excitation in which the transit time between gaps is about 1.25 cycles, and the. beam passes through the system once. and is collected on an electrode I0, the total frequency change available by changing the beam accelerating voltage is of the order of 0.6%. This is quite large compared to the frequency change in a normal single-transit oscillator with coaxial circuit closed at both ends as described in the aforementioned U. S. Patent No. 2,320,860, which is of the order of 0.05%, and gives a measure of the advantage gained by the use of the circuit structure described with reference to Figures 2 and 3.

As has been pointed out in the aforementioned patent, the electron discharge devices may in some circumstances be advantageously made to work by reflecting the beam back through system again, using the electrode Ill at or below cathode potential. In this case, and particularly if the transit time between the gaps 5 and 5 is about 1,/2 the frequency period or cycle and that between the gap 5 and the relection point is between and @Si available with change in accelerating voltage is far greater. Thus with this drift space length and a reflecting path length of about 'C78 cycle the frequency change at a mean irequency o1" 2,490 megacycies per second may be about l megacycle per second per volt at a nie-an of S50 volts or with a reflecting path oi 11/8 cycles may be 2.2 rnegacycles per second per volt at 500 volts, the total possible frequency swing being about 300 megacycles per second. The output efficiency in such a case would throughout the swing be over 1%, not varying by more than 2 to l, or in other conditions may give an appreciably higher efficiency, about 3-5% over a rather smaller frequency range.

In the practical form of construction shown in Figures 4, 5 and 6' the short length of coaxial line comprises an outer cylindrical member I3 and an inner or central conductor member M. The outer cylindrical member I3 comprises two channel shaped sections provided with integral gapdening ns I5 and welded or riveted together to form a tube, for example, as described in the specification of patent application No. 457,795 now Patent No. 2,451,328. The central conductor I4 which denes the drift space comprises a portion c having a cross-section equal to the interior cf the outer member I3, a portion b forming the internal conductor of the coaxial transmission line and a portion a having a slot therein for the passage of the electron beam, and shown more clearly in Figure 5. An antenna I-Ii is mounted on the end of the portion ct as shown in Figure 4. The portion c it will be observed closes the end of the chamber. The electron stream is produced by means of an indirectly heated cathode I1 and grid IB. A channel shaped anode i9 is provided and may perform the function of a collector electrode or a reflector for the electron beam as will be hereinafter explained. Accelerating potentials may be applied to the resonator. A shield 20 is mounted behind the cathode I'I. The cathode, grid and anode are mounted between and supported by two mica discs 2I and 22 and the upper end of the resonator I3 is closed by a copper disc 23 secured to a mica disc 24 and aids in supporting the antenna rod I5. A further mica disc 25 is provided at the lower end of the assembly which is mounted on a rectangular U-shaped bridge member 35 (Figure l2) and secured thereto by short projections 53 provided on the lower end of member I3 which are bent over so as to grip said bridge member. This latter is in turn mounted on a press in the usual manner, which press is sealed in the bottom of the glass envelope 26. Rods 2l, 28, 29 and 39 are provided and rod 2? which is electrically connected to the anode I9 is connected at its lower end to a connecting wire which passes through the press 35 (Figure S) and connected to one of the pins 37 in the base 35 (Fig. 6) of the tube in the manner usual in the art. Rod 3D is connected at its upper end to grid I8 and at its lower end is connected by a conducting wire to a pin in the base. Rod 29 is connected at its upper end to the shield 20 and at its lower end by a conducting wire to a Din in the base. Rod 28 is not connected to any pin and helps only to support the anode. Two wires 3i, 32 connected to two pins in the base and maintained separated by a mica bridge piece 33 serve to carry the heating current to the cathode heater. A connection from the resonator I3 and from the cathode are also brought outthrough I the press to respective pins in the base.

5 press and these rodsare-connectedto-"one of' the pins 37m the'basefso thata steadyfdirectfcurrent potential may be applied to the resonator serving, for example, to accelerate the electrons ofthe beam'.

In operation, the tube is mounted' between the polesof a magnet so thatthe tube cathode SYS;- tem is symmetrically placed' inthe `gap between the poles, and the magnetic iieldv directs; and concentrates the electron beam into the space between the pairs of ins 8 or 9 (Fig. 2) and through the drift tube orchannel 1*.

The tube may be operated so that the electron beam is reflected back from the anode and an adjustable negative voltage is necessary for the grid potential in order to prevent excessive dissipation. The anode may be strapped to the cathodev if the resonator is given a higher voltage. In one practical case, optimum results were `obtained when the anode potential was 60C- volts negative to the cathode. In order to in'- crease the power outpuhthe tube may be shielded, for example, by copper foil and the aerial circuit should vnot be too selective. The tube will operate satisfactorily when the antenna rod is radiating and a reiiector as shown at 4 in Figure 1 may be used to direct the radiations. Alternatively the tube may be inserted into a waveguide as-shown in Figure 7 in which the wave guide is designated 3S and is partially broken away to show the tube 26. The type of wave generated in the guide will, as will be understood by those versed in the art depend upon the relation of the antenna and the walls of the guide. The antenna may, for example, be located at the end of a guide with its coaxial with the axis of the guide. The guide may serve as the transmission medium for the wave-s to be received and heterodyned by the oscillations generated in the tube. The waves of intermediate lower frequency produced may then be received and used to derive a voltage proportional to the intermediate frequency which voltage is then applied to regulate the beam accelerating voltage of the tube so as to maintain the intermediate frequency constant. Arrangements for producing such a voltage are well known in the art and it is considered that no further description is necessary herein since they form no part of the present invention.

An alternative form of construction of the resonator is shown in Figures 8 and 9, in which the resonator is shown as formed from a stack of like apertured plates as described and claimed in the specification of U. S. Application Serial No. 457,789. The stack comprises alternate thick plates l0 of copper and thin plates 4I of molybdenum. The plates di differ from the plates le in that the barrier separating the two principal apertures is extended to form a long tongue 42, and the sides have extensions l5 having curved edges 53 concave to the tongue 42.

The tongues 42 of the molybdenum plates 4l are connected together by means of two piles of packing pieces 54A and MB, shown in Fig. 9 to form an antenna corresponding to i6 of Fig. 4. The pieces 64B are spaced apart from the plates 49 so as to form a number of small rectangular apertures 5d which together constitute a vertical slot through which a beam of electrons for exciting the device may be projected in the direction of the arrow 45, shown in Fig. 8, past the gaps 41 and 48 in the plates 40. The stack of plates is secured together by means of rods or bolts 49 passing through suitably located holes 43 in the plates, as will be clear. When the device is in operation,an alternating difference of potential-exists across each of the gaps `dfi-and .48 so that an alternating potential is applied to the antenna with respect to the extensions 45, which together form a concave reector for the waves radiated by the antenna. The means for'producing said beam of electrons may comprise an electron gun 5b, grid 5l, and a collector electrode 52, as described with reference to Figs. 10-and 1-1, the width of the beam being substantially the saine-as the depth of the stack. The stack and electron beam producing arrangement are mounted within a glass or other envelope which can be evacuated, and preferably with the antenna along the longitudinal axis of the tube as in Figs. 10'andA 11. Conducting leads from the electrodes and resonator to pins in the tube base are provided as indicated in connection with Fig-ure 4.

Whilst two embodiments of the invention have been described by way oi example, others falling within the scope of the appended claims Will occur to those skilled in the art. For example a further embodiment may comprise the electron discharge apparatus as described in application No. 476,465 filed February 19, 1943, by J. H.

Fremlin et al. with the proviso that the res-- onator chamber and antenna together must be of a type giving a wide range of. change of frequency with change of accelerating voltage.

What is claimed is:

1. An ultra-high frequency oscillator of the velocity modulation type comprising a source of an electron beam, means for controlling the mean velocity of the electrons in said beam, said source and said controlling means dening a beam path, a rst cavity resonator provided with an aperture therethrough mounted about said beam path adjacent said source for velocity modulating electrons in said beam, a second cavity resonator provided with an aperture therethrough mounted about said beam path adjacent said rst cavity resonator for collecting ultra-high frequency energy from the velocity modulated electrons, said cavity resonators both being provided with an open side, said cavity resonators being mounted with said open sides adjacent each other, and an antenna mounted on said resonators adjacent said open sides for capacitively loading said resonators, said antenna being less than a quarter wave length long at the highest operating frequency.

2. An ultra-high frequency oscillator of the Velocity modulation type as set forth in claim 1 in which said cavity resonators comprise a stack of metal plates, each of said plates being provided with two apertures and two slots extending from one edge thereof into said apertures and alternate ones of said plates have tongues extending from that part of each said plate which is between the slots through the edge of the plate; said tongues being connected at the external portion, whereby capacitative coupling between the two cavity resonators is provided.

3. An ultra high frequency oscillator of the velocity modulation type comprising a source of an electron beam, means for controlling the mean velocity of the electrons in said beam, said source and said controlling means defining a beam path, a cavity resonator of the coaxial line type provided with apertures on a diameter thereof and mounted with said apertures aligned with said source and said controlling means about said beam path, means closing one end of said coaxial line resonator, the other end thereof being open,

and an antenna connected to the inner line of the coaxial line type cavity resonator at the open end thereof, said antenna being less than one quarter of a wave length long at the highest frequency at which said apparatus is to operate for capacitively loading said resonator.

4. An ultra high frequency oscillator as set forth in claim 3 in which the means for controlling the mean velocity comprises means connected to said cavity resonator for applying thereto an accelerating voltage.

5. An ultra high frequency oscillator as Set forth in claim 3 further comprising a reflector plate attached to the cavity resonator at the open end for reilecting back radiation from the antenna.

6. An ultra high frequency oscillator of the velocity modulation type comprising a source of an electron beam, means for controlling the mean velocity of the electrons in said beam, said source and said controlling means defining a beam path, means mounted about said beam path adjacent said source for velocity modulating electrons in said beam, means mounted about said beam path adjacent said modulating means for collecting ultra high frequency energy from the velocity modulated electrons, said modulating and extracting means comprising a resonator system including at least one cavity resonator and an antennacoupled to said resonator` at a region of maximum high frequency potential therein for capacitively loading said resonator, said antenna being less than a quarter wave length long.

7. An oscillator according to claim 6, wherein said cavity resonator comprises a half wave length resonator closed at one end and open at the other, said antenna being mounted on said resonator at said open end.

J OHN HEAVER FREMLIN.

REFERENCES CITED The following references are of record in the le of this patent: 

